1. Field of the Invention
The present invention relates generally to the restoration of teeth, and more particularly, to methods and devices for improving the accuracy and simplifying the process of performing such restorations.
2. State of the Art
Presently, numerous methods exist for the restoration of teeth by dentists, including the use of artificial tooth material (such as gold or porcelain) to form a cast-restoration or a metal-ceramic restoration (i.e., dental prosthetics such as crowns). Prosthetic crowns are typically used to repair decayed tooth structure where support from the original tooth structure is either marginal, or unavailable.
Known techniques for preparing a tooth to receive a crown are susceptible to numerous variables, some of which are within the dentist's control and some of which are not. All of these variables can detrimentally influence the accuracy with which: (1) the tooth is prepared to receive the crown; (2) the crown is prepared for placement on the tooth; and (3) the manner by which the crown is fit to and fixed on the prepared tooth.
For example, in preparing the original tooth to receive the crown, a dentist will typically use various shaped diamond burs in a high speed hand tool to remove approximately 2 mm of exterior tooth structure. The coronal portion of the tooth is shaped so that when a prosthetic crown is received from a laboratory, it will be of the approximate size and shape of the patient's original coronal portion prior to preparation. Because tooth preparation is performed totally by manipulation of a hand tool, and because the skill required for such tooth preparation will vary among dentists, the precision with which a tooth is prepared will vary widely.
Further, the quality of the prosthetic crown will vary based on the skill of the person who actually produces the crown (e.g., laboratory technician). More particularly, after the patient's tooth has been shaped to receive the prosthetic crown, an impression is formed from the prepared tooth by placing impression material into the patient's mouth (i.e., to form a negative impression of the prepared and adjacent teeth). To accurately prepare the impression, all gingival bleeding must be stopped and the margin of the gum tissue must be retracted from the lower portion of the tooth. The impression material must then be properly injected into the sulcus area of the tooth. A tray which contains a combination of impression materials is then applied with pressure over the teeth in the area of the prepared tooth, including the prepared tooth.
Despite efforts by the dentist to obtain an accurate impression of the prepared tooth, many factors can detrimentally influence quality of the impression. For example, the ability of the dentist to maintain a dry field of operation in the area of the prepared tooth can inhibit accuracy of the impression. The retraction of the gingival tissue can also affect the accuracy of the impression, as can the dentist's technique in obtaining the impression (i.e., the general care in obtaining an accurate impression).
Once the impression has been produced by the dentist, a laboratory technician will set die pins in the impression and then form a master impression as a die (e.g., plaster models) of the patient's teeth. The technician will set the occlusal bite registration and articulate the models of the patient's teeth. Afterwards, the laboratory technician will saw the die to remove the tooth of interest, then trim the die of the tooth and mark the marginal finish line. The sub-structure is then waxed for preparation of the prosthetic crown.
After a wax pattern has been formed, it is converted (i.e., cast or machined) into a sub-structure (e.g., coping) of the crown. It is a challenge to produce a coping that will comply with acceptable tolerances, given the variables associated with the quality of the impression, the skill of the technician and the proper selection of die materials.
U.S. Pat. No. 5,135,393, assigned to Mikrona, describes a coping mechanism for producing parts such as non-metal copings. As described therein, a three-dimensioned pattern is sensed (e.g., traced) with a feeler pin, and then sensed deflections or displacements of the feeler pin are transferred to a motor driven machining tool. As the pattern is traced, the motor driven machining tool operates upon a blank to fabricate a matching three-dimensional coping. The coping is later used by the dental laboratory to build-up a finished crown.
That is, once the machined coping has been produced, it is processed with a porcelain build-up. The build-up material incorporates specific shading and color effects to simulate the enamel of the original tooth. The porcelain build-up is then vacuum fired.
The combination of producing a coping, followed by building-up the coping with porcelain, are thus required to produce the prosthetic crown. The final stages of crown preparation include finishing the porcelain build-up, after which the anatomy of the original tooth structure is carved therein. The porcelain crown is then glazed. Where the crown is formed of cast metal, the cast exterior of the crown is sand-blasted to remove external oxidation. The metal interior is then polished and the fit, shading and prosthetics of the crown are quality checked. The finished crown is then returned to the dentist for placement onto the prepared tooth structure.
Processes which involve using devices such as those described in U.S. Pat. No. 5,135,393 are not practical for widespread use in dentistry for a variety of reasons. These devices involve complex and timely processes for producing a finished prosthetic suitable for placement in a patient's mouth.
For example, to produce a finished crown, the process described in the '393 patent requires: (1) initially making a dental impression of the patient's teeth; (2) producing a hand made pattern (i.e., template), such as a template of a three-dimensional dental coping from the impression; (3) using an apparatus as described in the '393 patent to produce a non-metal coping by tracing the template and concurrently machining an oversized blank; (4) building-up the machined, non-metal coping in a dental laboratory with a crown material, such as porcelain; (5) sintering the crown material on the non-metal coping and returning the finished crown to the dentist for final adjustment and placement in the patient's mouth.
Thus, while an apparatus as described in the '393 patent is useful in machining dental parts, it does little to reduce the time and complexity associated with producing finished dental prosthetics such as crowns and bridges. The process of shipping an impression from the dentist's office to the laboratory technician, the preparation of the crown and the returning of the crown to the dentist typically involves a period of approximately two weeks. Upon receipt of the prosthetic crown from the laboratory, the dentist removes a temporary crown which had been placed over the prepared tooth of the patient following preparation of the impression. The permanent crown is then cemented into place. The dentist's skill is again called upon to ensure proper fit, occlusion bite registration and aesthetics of the prosthetic crown. While the dentist can modify the occlusion of the crown, inaccuracies in fit can require that a new crown be prepared and the entire process described above repeated, thus leading to increased time delays and patient discomfort due to prolonged use of a temporary crown. In some cases, if the crown does not accurately fit, the dentist will use a bur to ground the interior; however, the use of a bur to shape the crown interior alters the fit and therefore detrimentally affects the marginal seal.
Thus, it would be desirable to improve the accuracy with which tooth restorations are performed. Further, it would be desirable to reduce the skill-dependent tasks associated with tooth restoration, and to reduce the cost associated with such procedures, without compromising the quality of these procedures. Ideally, it would be desirable to provide a process which would enable a crown to be completely produced in a dental office, within the course of a day.